1. Field of the Invention
The present invention generally relates to a communication system with a diversity scheme, and more particularly to a communication system with an improved diversity scheme which uses weak electromagnetic waves as a medium and in which a transmitting station transmits the same data on carriers having different frequencies.
2. Description of the Related Art
Generally, radio communication makes use of electromagnetic waves of a specific frequency. Such a communication system is exposed to the influence of interference waves and that of fading. There is a communication system which is free of the influence of interference waves and fading. This system has a diversity scheme, wherein the same data is transmitted on a plurality of carriers of different frequencies.
FIG. 5 is a block diagram showing a known system with a diversity scheme. This system has a transmitting station and a receiving station. In the transmitting station, data D1 is input to a first transmitting section 1 and a second transmitting section 2. The first transmitting section 1 modulates the data D1, superposing it on a first carrier. The second transmitting section 2 modulates the data D1, superposing it on a second carrier. As a result, the first and second transmitting sections 1 and 2 produce two sets of data. The two sets of data are supplied via an antenna combining equipment 3 to an antenna 4, and are transmitted therefrom in the form of two streams of electromagnetic waves. The streams of electromagnetic waves travel through the air. In the receiving station of the communication system, an antenna 6 receives the two streams of electromagnetic waves transmitted from the antenna 4 of the transmitting station. The first stream of electromagnetic waves, i.e., the data D1 superposed on the first carrier, is input to a first receiving section 7. The second stream of electromagnetic waves, i.e., the data D1 superposed on the second carrier, is input to a second receiving section 8. The first receiving section 7 demodulates the first stream of electromagnetic waves into data, whereas the second receiving section 8 demodulates the second stream of electromagnetic waves into data. The data items output by the first and second receiving sections 7 and 8 are compared in terms of their qualities, and one of data of better quality is selected by means of a changeover switch 9 and output therefrom as data D2.
As can be understood from the above, a communication system with a frequency-diversity scheme, however, needs to have a plurality of transmitting sections, a plurality of receiving sections and the antenna combining equipment.